1. Field of the Invention
The invention relates to ink jet printing techniques and, more particularly, to a length mode, large array, high density, drop on demand type ink jet print head for use in ink jet printing applications.
2. Description of Related Art
Printers provide a means of outputting a permanent record in human readable form. Typically, a printing technique may be categorized as either impact printing or non-impact printing. In impact printing, an image is formed by striking an inked ribbon placed near the surface of the paper. Impact printing techniques may be further characterized as either formed-character printing or matrix printing. In formed-character printing, the element which strikes the ribbon to produce the image consists of a raised mirror image of the desired character. In matrix printing, the character is formed as a series of closely spaced dots which are produced by striking a provided wire or wires against the ribbon. Here, characters are formed as a series of closely spaced dots produced by striking the provided wire or wires against the ribbon. By selectively striking the provided wires, any character representable by a matrix of dots can be produced.
Non-impact printing is often preferred over impact printing in view of its tendency to provide higher printing speeds as well as its better suitability for printing graphics and half-tone images. Non-impact printing techniques include matrix, electrostatic and electrophotographic type printing techniques. In matrix type printing, wires are selectively heated by electrical pulses and the heat thereby generated causes a mark to appear on a sheet of paper, usually specially treated paper. In electrostatic type printing, an electric arc between the printing element and the conductive paper removes an opaque coating on the paper to expose a sublayer of a contrasting color. Finally, in electrophotographic printing, a photoconductive material is selectively charged utilizing a light source such as a laser. A powder toner is attracted to the charged regions and, when placed in contact with a sheet of paper, transfers to the paper's surface. The toner is then subjected to heat which fuses it to the paper.
Another form of non-impact printing is generally classified as ink jet printing. Ink jet printing systems use the ejection of tiny droplets of ink to produce an image. The devices produce highly reproducible and controllable droplets so that a droplet may be printed at a location specified by digitally stored image data. Most ink jet printing systems commercially available may be generally classified as either a "continuous jet" type ink jet printing system where droplets are continuously ejected from the print head and either directed to or away from the paper depending on the desired image to be produced or as a "drop on demand" type ink jet printing system where droplets are ejected from the print head in response to a specific command related to the image to be produced.
In a continuous jet type ink jet printer, a pump supplies ink to a nozzle assembly where the pumping pressure forces the ink to be ejected therefrom in a continuous stream. The nozzle assembly includes a piezo crystal continuously driven by an electrical voltage, thereby creating pressure disturbances that cause the continuous stream of ink ejected therefrom to break up into uniform droplets of ink. The droplets acquire an electrostatic charge due to the presence of an electrostatic field established close to the ejection orifice. Using high voltage deflection plates, the trajectory of selected ones of the electrostatically charged droplets can be controlled to hit a desired spot on a sheet of paper. The high voltage deflection plates can also deflect unselected ones of the electrostatically charged droplets away from the sheet of paper and into a reservoir for recycling purposes. Due to the small size of the droplets and the precise trajectory control, the quality of continuous jet type ink jet printing systems can approach that of formed-character impact printing systems. However, one drawback to continuous jet type ink jet printing systems is that fluid must be jetting even when little or no printing is required. This requirement degrades the ink and decreases reliability of the printing system.
Due to this drawback, there has been increased interest in those printing systems in which droplets are ejected from the print head by electromechanically induced pressure waves. In this type of printing system, a volumetric change in the fluid is induced by the application of a voltage pulse to a piezoelectric material which is directly or indirectly coupled to the fluid. This volumetric change causes pressure/velocity transients to occur in the fluid, thereby causing the ejection of a droplet therefrom. Since the voltage is applied only when a droplet is desired, these types of ink jet printing systems are referred to as "drop on demand" type ink jet printing systems.
A typical drop on demand type ink jet printing system is disclosed in U.S. Pat. No. 3,946,598 to Kyser et al. In Kyser et al., a pressure plate formed from two transversely expandable piezoelectric plates is utilized as the upper wall of an ink-carrying pressure chamber. By applying a voltage across the piezoelectric plates, the pressure plate flexes inwardly into the pressure chamber, thereby causing a fluid displacing volumetric change within the chamber.
Another typical drop on demand type ink let printing system is disclosed in U.S. Pat. No. 4,536,097 to Nilsson. In Nilsson, an ink jet channel matrix is formed using a series of piezoelectric strips disposed in spaced parallel relationship with each other and covered by a plate on both sides. One plate is constructed of a conductive material and forms a shared electrode for all of the strips of piezoelectric material. On the other side, electrical contacts are used to electrically connect channel defining pairs of the strips of piezoelectric material. When a voltage is applied to the two strips of piezoelectric material which define a channel, the strips become narrower and higher such that the enclosed cross-sectional area of the channel is enlarged and ink is drawn into the channel. When the voltage is removed, the strips return to their original shape, thereby reducing the channel volume and ejecting ink therefrom. Other, albeit shear mode type, ink jet printing systems which utilize separate sections of a piezoelectric material to form individual actuator walls for an ink-carrying channel are disclosed in U.S. Pat. Nos. 4,879,568 to Bartky et al. and 4,887,100 to Michaelis et al.
The major drawback to drop on demand type ink jet printing systems such as those disclosed in Kyser et al., Nilsson, Bartky et al. and Michaelis et al. is the difficulty in manufacturing an ink jet print head, particularly a large array or high density ink jet print heads, in such configurations. Each of these configurations utilize a separate actuator piece for each channel. Accordingly, to construct such a print head, a large number of individual parts must be used to assemble the channel array. Electrical contacts are either separately attached to each actuator before assembly or attached after the channel array is assembled, either of which is a very time consuming operation. Due to the large number of steps required to assemble such a print head, the manufacture of such a print head with a nozzle density greater than 100 nozzles per inch has proven difficult in practice.
Shear mode type piezoelectric transducers which are common to multiple ink-carrying channels in a drop on demand type ink jet printing system are disclosed in U.S. Pat. Nos. 4,584,590 and 4,825,227 to Fischbeck et al. In both of the Fischbeck et al. patents, a series of open ended parallel ink pressure chambers are covered with a sheet of a piezoelectric material along their roofs. Electrodes are provided on opposite sides of the sheet of piezoelectric material such that positive electrodes are positioned above the vertical walls separating pressure chambers and negative electrodes are positioned over the chamber itself. When an electric field is applied across the electrodes, the piezoelectric material, which is poled in a direction normal to the electric field, distorts in a shear mode configuration to compress the ink pressure chamber. In these configurations, however, three or more electrodes are required for each channel. Furthermore, as respective deflections at various locations along a single sheet of piezoelectric material is used to activate the various channels included in the array, adjacent channels must be spaced a considerable distance apart or use a separate restraining mechanism to prevent cross-talk between adjacent channels. For these reason, the Fischbeck et al. configurations are not particularly well suited for large array or high density applications.
By allowing the manufacture of large array, high density, drop on demand type ink jet print heads, the present invention will enable the design and manufacture of various printing systems, including printers, facsimile machines, copiers and others, in both single multiple color applications, with higher performance characteristics and at lower cost, than many existing printing systems.